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Lecture 8
Wireless Sensor Networks:
Overview
Reading: • “Wireless Sensor Networks,” in Ad Hoc Wireless Networks: Architectures and Protocols, Chapter 12, sections 12.1-12.2.
I. Akyildiz, W. Su, Y. Sankarasubramaniam and E. Cayirci, “A Survey on Sensor Networks,” IEEE Communications Magazine, August 2002. - D. Estrin, R. Govindan, J. Heidemann, and S. Kumar, "Next Century Challenges: Scalable Coordination in Sensor Networks," Proc. Mobicom '99, August 1999. - “Wireless Sensor Network Protocols” by M. Perillo and W. Heinzelman. To appear in: Fundamental Algorithms and Protocols for Wireless and Mobile Networks, CRC Hall, 2005.
Wireless Sensor Networks (WSNs)
Microsensors Low power, cheap sensors Sensor module (e.g., acoustic, seismic, image) A digital processor for signal processing and network protocolfunctions Radio for communication Battery-operated Sensors monitor environment Cameras, microphones, physiological sensors, etc. Gather data for some purpose Hundreds or thousands of nodes scattered throughout an environment Each sensor can collect data Data routed via other sensors to a sink or base station node
WSN Applications
New wireless networking paradigm
Requires autonomous operation
Highly dynamic environments
Sensor nodes added/fail
Events in the environment
Distributed computation and communication protocols required
Applications
Home security
Machine failure diagnosis
Chemical/biological detection
Medical monitoring
Surveillance and reconnaissance
Animal/plant monitoring (e.g., for research)
Example Application:Environmental Monitoring
Traffic patterns many-to-one Raw sensor data or highlevel descriptions aboutenvironmental phenomena Example projects
ZebraNet
Ecology of rare plants inHawaii
Sensor Platforms
Example platforms Smart Dust (UC Berkeley) Berkeley Motes Telos Motes (MoteIV) iBadge (UCLA) WINS (UCLA)
Sensor Platforms
WSN Limitations (cont.)
Communication
The bandwidth is limited and must be shared among all thenodes in the sensor network
Spatial reuse essential
Efficient local use of bandwidth needed
WSNs vs. MANETs
Many-to-one traffic pattern One-to-one traffic pattern Data-centric Address-centric Very constrained energy and bandwidth Constrained energy and bandwidth Cooperative Competitive Application-specific QoS QoS: delay, etc Typically immobile Typically mobile Large-scale Small-scale Require self-configuration Require self-configuration Unreliable communication Unreliable communication
Sensor Networks
General Ad Hoc Networks
Design Factors (cont.)
Topology
Deployment: random or deliberate placement of nodes Changes in topology during network operation
New nodes added to the system
Nodes failing
Environmental changes
Energy consumption
Sensor functions: sensing, communication, dataprocessing All require energy
Evaluating WSNs
What are the performance metrics for WSNs? System lifetime
E.g., time until network partition
E.g., time until probability of missed detection exceeds athreshold
Quality of result of sensor network
Application-specific measure
Latency of data transfer
SNR of aggregate data signal
Probability of missed detection or false alarm
Tradeoffs can be made among network parameters
E.g., can reduce quality of result of sensor network to increasesystem lifetime
Taxonomy (cont.)
Sensor resources
Memory Processing Transmit power (fixed vs. variable) Locations/density
Traffic patterns
Event-driven applications Continuous data generation Query-driven applications
Design Issues
New protocols needed
MAC
Cooperative nature of sensor networks (fairness not an issue) Exploit traffic patterns Energy efficiency extremely important Reduce idle listening Reduce unnecessary reception
Routing
Different traffic models Data dissemination rather than point-to-point routing Data-centric rather than address-centric Location-aware sensors Resource-aware routing needed Exploit local aggregation
Design Issues (cont.)
Time synchronization
Very important in sensor networks
Needed to determine if event sensed by two sensors is in factthe same event
Needed to determine object speed
Approaches
GPS – expensive, not energy-efficient
NTP (used in computer networks) – not enough precision
Newer approaches being researched
Romer’s Algorithm Reference-Broadcast Synchronization (RBS)
Design Issues (cont.)
Localization Important for same reasons as time synchronization Often times, only relative position is necessary GPS is overkill and unattractive for energy reasons RSSI used to infer distances Time of Arrival (ToA) Time Difference of Arrival (TDoA) Angle of Arrival (AoA) Sensor can find its own location using received beacons Sensor can have other nodes measure its location Sensor sends beacon message and neighbors use trilaterationbased on signal strength measurements Problem – small scale fading
